Automatic telephone system



Feb. 6, 1940. c. E. LoMAx ET AL AUTOMATIC TELEPHONE SYSTEM Filed April 25, 1938 5 sheets-sheet 1 ATTORNEYS.

c. E. LoMAx E1- AL 2,189,400

AUTOMATIC TELEPHONE SYSTEM Filed April 25, 193s 5 sheets-sheet s Feb. 6, 1940.

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AUTOMATIC TELEPHONE SYSTEM Filed April 25, 1938 5 Sheets-Sheet 4 TRUNK T0 CONN.

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Feb. 6, 1940.

' C. E. LOMAX ET AL Filed April 25, 1938 5 Sheets-Sheet 5 ooo l i C642 @so l 1/ M644 G9 G76 C645 gif-:.643 C 0 l; 684

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Patented Feb. 6, 1940 UNITED STATES PATENT OFFICE AUTOMATIC TELEPHONE SYSTEM Delaware Application April 25, 1938, Serial No. 204,086

29 Claims.

The present invention relates to automatic telephone systems and has for one of its objects the provision of improved switching apparatus for use in setting up connections between the various 5 lines of the system.

Automatic telephone systems conventionally comprise a plurality of subscriber lines provided with substations adapted to be connectedl thereto and an exchange housing switching apparatus including a plurality of subscriber controlled automatic switches adapted to be connected in tandem-for setting up connections between the various subscriber lines. More particularly, the switching apparatus may include a group of l finder switches and a group of connector switches having access to each subscriber line and groups of selector switches arranged between the groups of nder and connector switches and adapted to be connected thereto by trunk lines in order to forward the connection therebetween. Also, each subscriber line is provided with an individual line circuit for controlling the connections of the line to the finder and connector switches. Each line circuit and automatic switch embodies one or more relays provided with circuits including the conductors of the trunk lines, the circuits of the relays -being selectively controlled in order to eiect the desired operations of the line circuits and automatic switches. In a conventional telephone system of the type described, some of the control circuits of the various relays in the line circuits and automatic switches are so lconnected and arranged that potential difference is presentbetween the contacts of the contact sets in the bank contacts of the automatic switches terminating the subscriber lines and the trunk lines even when the subscriber lines associated with the line circuits and the automatic switches are idle. Under certain operating conditions, it is desirable that no potential difference is present between the contacts of the contact sets in the bank contacts of the automatic switches, when the subscriber lines and automatic switches are idle.

In a telephone system of this type, an established connection between two substations is ordinarily cleared out and the apparatus involved in establishing the connection is released under the control of one or both of the parties at the substations upon the performance of release operations thereat. In some instances, when `the release operations are performed, all of the apparatus involved in establishing the connection is not immediately released Linder certain specic release conditions, due to the interaction between certain of the control relays embodied in the apparatus. For example,v it has been found that, following the performance of the release operations, the control relays do not always restore simultaneously or in the same sequence and, when the control relays restore' in a predetermined se- 5 quence, an undesirable operating circuit for certain of the control relays is completed, which causes the relays to hunt. As a result of this hunting of the control relays, at least a portion of the apparatus involved in establishing the conl0 nection may be prevented from releasing properly.

It is an object of the present invention to provide an improved automatic telephone system of the type briefly described in which no potential difference is present between the contacts of the l5 Contact sets in the bank contacts of the automatic switches terminating idle subscriber lines and trunk lines eiitending to idle automatic switches.

Another object of the invention is to provide an improved automatic telephone system of the 20 character noted which embodies an arrangement for positively preventing any interaction between the various control relays in the line circuits and automatic switches following the performance of a release operation at a substation to clear out 25 an established connection, thereby to prevent the operated switching apparatus from being held operated.

A further object of the invention is to provide in an automatic telephone system of the type 30 noted an improved line circuit which is connected and arranged toprevent the presence of potential between the contacts of the contact sets in the bank contacts of the nder and connector switches terminating the associated subscriber 35 line when the subscriber line is idle.

A further object of the invention is to provide in an automatic telephone system of the type noted an improved automatic switch which is constructed and arranged to prevent the presence of potential between the contacts of the contact sets in the bank contacts of the preceding automatic switches having access to the automatic switch when the automatic switch is idle.

A'further object of the invention is to provide 5 inan automatic telephone system of the type noted an improved line circuit which is connected and arranged positively to prevent interaction between the control relays therein and the control relays in the automatic switches involved in an established connection following the performance of a release operation to clear out the established connection.

A further object of the invention is to provide in an automatic telephone system of the type noted an improved automatic switch which is connected and arranged positively to prevent interaction between the control relays therein and the control relays in the line circuits involved in an established connection following the performance of a release operation to clear out the established connection.

The features of the present invention are illustrated as being incorporated in an exchange having a capacity of ten thousand lines and including groups of finder, first selector, second selector and connector switches. The various lines individually terminate in line circuits for controlling the connections of the lines to the finder and connector switches. In accordance with one feature of the invention, each subscriber line is normally disconnected from the associated link and connecting lines terminating, respectively, at the bank contacts of groups of finder and connector switches and is connected thereto only when the subscriber line is busy, thereby to prevent the presence of potential between the contacts of the contact sets in the bank contacts of the finder and connector switches respectively terminating the link and connecting lines associated with the subscriber line, when the subscriber line is idle. More specifically, the associated subscriber, link and connecting lines each comprise two line conductors, the line conductors of the connecting line being directly connected to the corresponding line conductors of the link line, and the line conductors of the subscriber line being connected to the corresponding line conductors of the link line when the subscriber line is busy. In one form of the improved line circuit, one of the line conductors of the subscriber line is directly connected to a corresponding one of the line conductors of the link line, while the other line conductor of the subscriber line is connected to the other linel conductor of the link line under the control of a combined line and cut-off relay. In another form of the improved line circuit, both of the line conductors of the subscriber line are normally disconnected from the corresponding line conductors of the link line and are connected thereto under the control of a cut-olf relay.

In accordance with another feature of the invention, both forms of the line circuit are provided with means for preventing the interaction of a certain control relay therein and a certain control relay provided in at least one of the selector switches, following the performance of a switch train release operation performed at a substation connected to a calling line, which interaction, if allowed to occur, is effective to prevent the immediate release of the corresponding line circuit and selector switch. More specifically, each of the line circuits embodies contacts controlled by a relay embodied therein and operative in response to the performance of a release operation at a substation connected to a calling line, which contact interrupt an undesirable operating circuit f r the relays noted, thereby to prevent the interaction of the relays.

In accordance with a further feature of the invention, each of the second selector and connector switches is provided with means for preventing the presence of a potential between the contacts of the contact sets in the bank contacts of the preceding selector switches. This is laccomplished by providing in each of the second selector and connector switches va relay operative to initiate operation of the switch,

which relay is provided with an operating circuit including one terminal of a source of supply derived in the switch, a series connection between the line and control conductors of a trunk line incoming to the switch and the other terminal of the source of supply derived in a preceding switch having access to the switch, whereby no potential is present between the conductors of the trunk line incoming to the selector or connector vswitch until after the trunk line is seized by the preceding switch.

In accordance with a further feature of the invention, each of the second selector switches is provided with meansffor preventing interaction between a certain control relay 'provided therein and one of the relays provided in one of the line circuits associateds respectively with two connected lines following the performance of a release operation at the substation connected to the calling one of the lines, which interaction, as previously noted, is effective to prevent the immediate release of the corresponding line circuit and selector switch. In two different forms of the second selector switch, this is accomplished by providing a set of contact springs in the previously noted undesirable operating circuit, which contact springs are controlled to' interrupt the undesirable operating circuit by a predetermined operation of the wiper operating mechanism embodied in the switch, the predetermined operation of the mechanism being incident to the performance of the release operations. In a third form of the selector switch an uni-directional conducting device in the form of a copper oxide rectifier is included in the undesirable operating circuit, which device blocks the ow of current through the undesirable operating circuit, thereby to cause the restoration of the control relays noted and the consequent release of the corresponding line circuit and selector switch.

Further features of the invention pertain to the particular arrangement of the circuit elements, whereby the above-outlined and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specication, taken in connection with the accompanying drawings, in which Figure 8 illustrates the general arrangement of an automatic telephone system having incorporated therein apparatus constructed and arranged in accordance` with the present invention; Figs. 1, 2 and 3, taken together, illustrate the details of the automatic telephone system of Fig. 8; Fig. 4 illustrates a modified form of the selector switch of Fig. 2; Fig. 5 illustrates an-v other modied form of the selector switch of Fig. 2; Fig. 6 illustrates a further modified form of the selector switch of Fig. 2; and Fig. 7 illustrates a modified form of the line circuit of Fig. l.

Referring now more particularly to' Fig. 8 of the drawings, there is illustrated the general ar'- r rangement of the telephone system sh'wn in Figs.

1, 2 and 3. In brief, the system of Fig. 8 com-v prises an exchange having extending thereto a plurality of subscriber lines, two of which are indicated at Illl and IUS, and a plurality of subscriber controlled automatic switches adapted to be connected in tandem and including a line nder switch |82, a first selector switch |00, a

second selector switch 200 and a final selector or connector switch 300, for setting up connections between the various subscriber lines. It will be understood that a plurality of switches of each type, namely, a plurality of line finder switches, a plurality of first selector switches, a plurality of second selector switches, and a plurality of final selector or connectoi` switches, are provided in the system for performing the usual line switchingfunctions. The switches of each type are respectively arranged in groups and, in accordance with universally followed practice, the number of switches in each group is determined s in accordance with the maximum volume of traffic the particular switch group is required to handle. Specifically, the exchange is designed to serve ten thousand subscriber lines, these lines being divided into one hundred-line groups with the conductors of the lines of each group terminating in the bank contacts of a group of finder switches and a group of connector switches. In accordance with. conventional practice, each line is multipled between the'corresponding bank contacts of a group of nder switches and the corresponding bank contacts of a group of connector switches, whereby a plurality of such switches are available for setting up connections between each line and every other line of the system. Thus, the conductors of the line |0| are adapted to be connected by way of a line circuit |00 to the bank contacts of the finder switch |82,

which is one of a group of finder switches for serving the group of lines including the line |0|. This line, in addition to being multipled in the bank contacts of the finder switches in the group noted, is also multipledin the bank contacts of a group of connector switches. Similarlyy the line |09 is adapted to be connected by way of a line circuit |8| to the bank contacts of the connector switch 300, and is multipled between the corresponding contacts of other connnector switches of the group of connector switches including the connector switch 300 and the corresponding bank contacts of the group of nder switches including the linder switch |82.

The end of each subscriber line remote from the exchange housing the switching apparatus illustrated terminates in a subscriber substation which may be of any desired arrangement. Two such substations are diagrammatically illustrated at A and B associated with the lines |0| and |09, respectively. In brief, each of the substations may comprise the usual hand set type of transmitter and receiver, a hook switch, a ringer, and an impulsing device for transmitting the switch control impulses by way of the associated line to the automatic switching equipment embodied in the line terminating exchange.

As indicated above, each subscriber line has associated therewith a group of line finder switches, any one of which is capable of establishing a link connnection between the subscriber line and an associated rst selectorlswitch, each finder switch and its associated /first selector switch constituting a finder-selector link. In order, therefore, to assign the finder-selector links constituting a particular group to successive calling subscriber lines included'in the associated line group, :there are provided distributors, associated with the group of finder switches including the switch |82 associated with the first selector switch |00 being indicated generally at |83. Since this distributor may be of any conventional arrangement, it has been only diagrammatically illustrated in the drawings.

Each of the line finder, first selector, second selector, and connector switches is preferably of the well-known Strowger type having embodied therein a wiper carriage structure adapted to be translated vertically to bring the wipers thereof opposite a particular level of bank contacts, and then rotated to bring the contacts into engagement with a particular set of contacts in a selected level; vertical, rotary and release magnets; and control relays suitably connnected and arranged to control the energization of the various magnets and the line switching in accordance with particular functions of the switch. Obviously, the particular arrangement of the control apparatus as embodied in the switches of different type is different in each instance, depending upon the character of the switch, as is more fully explained subsequently. Since the nder switches are of conventional construction and well understood, the nder switch |82 has been only diagrammatically illustrated.

Each of the line circuits comprises a combined line and cut-off relay of the two-step type, two of which are shown at R|'10 and R|65 as being respectively embodied in the line circuits |80 and |8I. Each of the rst selector switches included in the system may be identical with that indicated generally at |00 in Fig. 1. In brief, this switch comprises a switching mechanism, indicated at |29, of the well-known Strowger type including a plurality of bank contacts arranged in levels; wipers for engaging the bank contacts;

.means comprising a vertical operating magnet M|84 for stepping the wiper carriage structure vertically to position the wipers oppositeV a particular selected level; means comprising a rotary operating magnet MI 85 for imparting rotary motion to the wiper carriage structure, thereby to movevthe wipers into engagement with a selected contact set in the selected level; and means comprising a release magnet M|86 for restoring the wiper carriage structure to its normal position during the release of the switch. Also incorporated in the selector switch |00v are the usual line and hold relays R|20 and R|30, respectively, a switching through relay R|50, circuit controlling relays R|40 and R|60, and rotary and Vertical off-normal springs indicated respectively at |81 and |91.

Each of the second selector switches included in the system may be identical with that indicated generaly at 200 in Fig. 2. In general, this switch is of the same construction as the rst selector switch |00 and comprises a switching mechanism, indicated at 2|8, of the well-known Strowger type including a plurality of bank contacts arranged in levels; provided with wipers for engaging the bank contacts; means comprising a vertical operating magnet M260 and a rotary operating magnet M26| for imparting the respective movements to the wiper carriage structure embodied in the switching mechanism; and means comprising a release magnet M262 for restoring `the wiper carriage structure to its normal position during the release of the switch. Also incorporated in the selector switch 200`are the usual line and hold relays R2|0 and R220, respectively, a switching through relay R240, circuit controlling relays R230 and R250, and rotary and vertical off-normal springs indicated respectively at 263 and 269.

Each of the final selector or connector switches included in the system may be identical with that indicated generally at 300 in Fig. 3'. In

To l

brief, this switch `comprises a switching mechanism indicated at 399 of the well-known Strowger type including a plurality of bank contacts arranged in levels; a carriage structure provided with wipers for engaging the bank contacts; means comprising a vertical operating magnet M390 for stepping the wiper carriage structure vertically to position the wipers opposite a particular selected level; means comprising a rotary operating magnet M391 for imparting rotary motion to the wiper carriage structure, thereby to move the wipers into engagement with a selected contact set in the selected level; and means comprising a release magnet` M392 for restoring the wiper carriage structure to its normal position during the release of the switch. Also incorporated in the connector switch 300 are the usual line, hold and battery reversing relays R320, R330 and R310, respectively, a ring cut-off relay R350, a busy relay R360, a switching through relay R380, circuit controlling relays R340 and R310 and two sets of vertical oli-normal springs indicated at 305 and 393.

Referring now more particularly to the operation of the apparatus illustrated in Figs. 1, 2 and 3, when a subscriber, such, for example, as the subscriber at the substation A, initiates a call over the line 101 to a second subscriber substation, such, for example, as thel substation B, by lifting his receiver from its hook, a circuit is completed for energizing in series the three windings of the combined line and cut-off relay R110. This circuit extends from ground at the upper terminal of the lower winding of R by way of the lower winding of R110, RA112, the line conductor C103 of the line -101, the bridge across C103 and C102 at the substation A, C102, RA111, the upper winding of R110, the control conductor C101, and the intermediate winding of R110 to battery. Due to the high series resistance of this circuit, the relay R110 only partially operates to close the X contacts associated respectively with RA114 and RA`115. At -RA115, ground potential at RA116 is applied to the distributor start conductor C116, thereby to cause the distributor 183 to initiate the operation of one of the finder switches to search-for the calling line 101. At RA114 a path is prepared for apply'- ing ground potential to the control conductor C101 of the line 101, thereby to mark the line 101 as busy in the bank contacts of the connectoi` switches at which this line terminates. Battry potential as applied to the lower terminal of the intermediate Winding of R110 is also extended by way of RA114 to the control conductor C105 ter',- minating in the bank contacts of the finder switches, thereby to mark the line 101 as the calling line. It is noted that the distributor 183 preferably functions in a manner such that an idle line finder switch is conditioned immediately to start operating in search of a calling line upon the application of ground potential to the start conductor C116. Assuming, for example, that the iinder switch 182 is the switch assigned by the distributor 183 to the use of the calling line 101, the wipers of this switch are rst operated to a position opposite the level in which the calling line is terminated, this level being identified through the application of 'marking or ground potential to the start conductor C116. When this level is found, the wipers are automatically stepped into the bank contact field to i test the individual lines terminating at the contact sets of the selected bank -contact level.

available idle line finder switch. Also in the course of these operations, direct ground potential is applied in the finder switch 102 to the control conductor C105, thereby to mark the calling line as busy through the application of ground potential to the control conductor C101 of the line 101 and to cause the full energization of the intermediate winding of R110. Asa result, the relay R110 now fully operates to remove, at RA116, ground potential from the start conductor C116 and to disconnect its windings from the line conductors C102 and C103 of the line 101. Thus, the calling linel is cleared of any impedance elements which might interfere with the transmission of irnpulsing and voice frequency currents. Also, upon fully operating, the relay R110 completes, at RA113, a connection between the line conductor C103 of the line 101 terminating at the substation A and the line conductor C104 of the line 101 terminating in the bank contacts of the finder switch 192, thereby to complete a link connection between the substation A and the first selector switch 10|. The line comprising the conductors C102, C104 and C105 and terminating'in the bank contacts of the finder s itch 182 also constitutes a. link line. If desired, oth of the line conductors C102 and C104 may bem ally disconnected from the line conductors C1 2 and C103 of the sub. scriber line 101 and connected thereto by the relay R110, as pointed out in connection with the modified form of line circuit 109 of Fig. 7.

Immediately the link connection is established between the substation A and the first selector switch 100, a circuit is completed for energizing the line relay R embodied in this switch. This circuit extends from ground on the dial tone conductor C by way of the contact springs 191 and 192 of the rotary off-normal springs 101, the

lower winding of R120, RC159, RA150, the line conductor C121 of the trunk line 124, the finder switch 102, C104, RA113, C103, the bridge across C103 and C102 at the substation A, C102, the finder switch |82, C125, RA151, RC152 and the upper winding of R120 to battery. Dial tone current is conducted over a similar path to that just traced and through the receiver of the telephone instrument at the substation A, thereby to indicate to the calling party that the dialing operations may be performed. Upon operating, the line relay R120 completes, at RA121 and RC122, a circuit for energizing the hold relay R130, this circuit extending from ground at RA151 by way of RA121, RC122 and the winding of R130 to battery. The last-mentioned relay, in turn, operates to complete, at RA131, an obvious path for applying multiple ground potential to the release conductor Cl26 of the trunk line 124, thereby to hold the finder switch 182 operated and to retain ground potential upon the control conductor C101 of the calling line 10| after ground potential in the iinder switch 102 is removed. This ground potential applied to the release conductor C|26 also retains the relay RHI) of the line circuit |80 in operated position. At RA|32 and RCI33, an impulsing circuit is prepared for energizing in series the control relay RMU andthe vertical magnet M|84 in accordance with the impulses comprising the first digit dialed at the calling substation A.

The irnpulsing` circuit for the control relay RI40 and the vertical magnet M|84 is completed. at the beginning of the rst impulse of the'rst series of impulses dialed at the calling substation A and when the line relay R|2IJ restores. This circuit extends from ground at RA|51 by way of RAI2|. RC|23, RAI32, RCl33, the Winding of RMD and the winding of MIM to battery. When energized over the above-traced circuit, the vertical magnet M|84 lifts the wiper carriage structure of the mechanism |29 one step or to a position opposite the first level of bank contacts. Upon operating. the control relay RIM] prepares at RAMI and RA|42 a circuit, traced hereinafter. for energizing the control relay RISO and interrupts at RAIIIZ and RCI43 a point in the circuit for energizing the rotary magnet M|85. Simultaneously with this operation, the vertical off-normal springs |91 are operated to the off-normal position, thereby completing at the contact springs |93 and |94 the circuit for energizing the control relay R 60, this circuit extending from ground at RAI3| by way of RAIBZ. RAMI, the contact springs |93 and |94 and the Winding of RISO to battery. At the contact springs |95 and |96 of the vertical off-normal springs |91, a ciri cuit traced hereinafter, is prepared for energizing the release magnet M|86. During each impulse of the iirst series of impulses. the vertical magnet MIM is energized in the above-described manner so that. at the conclusion of the rst digit, the wiper carriage structure ofthe mechanism |29 stands in a position opposite the level of bank contacts terminating the group of trunks outgoing to the second selector switches having access to the connector switches at which the called line |89 terminates. During the first series of impulses. each time the bridge across the line conductors C|02 and CI03 at the substation A is interrupted. the line relay R| restores, causing the control relay RMU and the vertical magnet Mlll to be energized, as previously explained. Also, each time the bridge across the line conductors C|ll2 and C|93 at the substation A is completed. the line relay R|20 reoperates and completes the circuit for energizing the hold relay RI30. Thus, thek hold relay R439 and the control relay RIM) remain operated during the impulsing period. due to their slow-to-release characteristics. Upon operating, the control relay R|69 prepares at RAIBI a point in the circuit for energizing the rotary magnet MI85 and completes at RA|62 a locking circuit for itself. the latter circuit extending from ground at RAIS'I by way of RA 62, the contacts controlled by the armature |85' of the rotary magnet M|85, the contact springs |93 and |94 and the winding of Rlll to battery.

With the wiper carriage structure of the switching mechanism |29 standing opposite the level of bank contacts terminating the group of trunks leading to the respective second selector switches of the group including the second selector switch 209, the first selector switch |80 is automatically conditioned to search for an idle one of these trunks. Thus, shortly following the end of the first series of impulses, the control relay RMU restores to interrupt, at RAMI and RA|42, the original operatingcircuit ofy the control relay RI6|I and to complete, at RAMZ and RCM3, the

circuit for energizing the rotary magnet MISS, the latter circuit extending from ground at RA|3| by Way of RAMZ, RC|43, RA|6I and the winding of M|85 to battery. Upon being energized over the above-traced circuit, the rotary magnet M|85 steps the wipers of the mechanism |29 into engagement with the first set of contacts in the selected level and simultaneously interrupts, at |85', the locking circuit of the control relay R|60, which latter relay restores to interrupt, at RA|6 I, the operating circuit of the rotary magnet M|85 and, at RA|62, a further point in its locking circuit. When the rotary magnet M|85 restores, it prepares a point, at |85', in a circuit for testing the idle or busy condition of the trunk line terminating at the engaged irst set of contacts in the selected level. If this first trunk line is busy, ground potential appears upon the release conductor thereof, thereby causing the control relay RiB to be reoperated, this circuit extending from the grounded release conductor by way of the conductor CISG, RAIS, RC|55, the armature |85', the contact springs |93 and |99 and the winding of R|68 to battery. It is noted that the ground potential appearing uponthe conductor CI36 and extending by way of RAIEA and RC|55 to the upper terminal lof the switching through rel ry RI 5U causes this relay to be short-circuited, since the lower terminal of the winding thereof is connected. by way of the contact springs |89 and |88 of the rotary oif-normal springs |81 to the grounded release conductor CIZS. Thus, operation of this relay is prevented. Upon rfoperating, the control relay RIGU completes, at RA|6| the circuit for energizing the rotary magnet MI85 and completes, at RAI62, its locking circuit. Upon operating, the rotary magnet M|85 steps the wipers of the mechanism |29 into engagement with the second set of contacts in the selected level and simultaneously interrupts at |85 thelocking circuit of the control relay RIBU, whereupon R|69 restores and interrupts, at RAISI, the operating circuit of the rotary ma-gnet- M|85 and interrupts, at RAI82, a further point in its locking circuit. The rotary magnet M|85 then restores and completes, at |85', the circuit for testing tn e1 idle or busy condition of the trunk line terminating at the second set of contacts in the selected level. If the second trunk is busy, ground potential appears upon the release conductor thereof, causing the control relay Rl to be reoperated, as previously explained, in order to cause the rotary magnet Mi 85 to be reoperated, thereby stepping the wipers of the mechanism |29 into engagement with the third set of contacts in the selected level. Thus, the rotary magnet M|85 operates to rotate the wipers of the mechanism |29 step by step over the contact sets in the selected level until they are moved into engagement with a trunk line extending to an idle second selector switch.

Assuming that the second selector switch 299 is the iirst idle selector switch in the selected level, when the wipers of the mechanism |29 engage the set of contacts terminating the trunk line 2M extending to the switch 200, further operation of the rotary magnet M|85 is interrupted, as no potential whatever appears upon the release conductor C2|6 of the trunk line 2M. Thus, the switching through relay R|50 is not short-circuited but is energized in series with the control relay R|6||, this circuit extending from the grounded release conductor C|26 by way of the contact springs |88 and |89, the winding of RI50, the amature |85', the contact springs |93 and |94, and the winding of RISO to battery. When the switching through relay R|50 is energized over the above-traced circuit, it is operated; while the control relay R|60, which is of the marginal type, is not operated, due to the high series resistance of this circuit. Upon operating, the switching through relay RI50 connects the line conductors C|25 and CI21 of the trunk line |24 to the line conductors C|35 and CI31,'re spectively, and connects the release conductor VC|26 of the trunk line |24 to the conductor C|36. At this time, the trunk line |24 extending to the rst selector switch |00 is connected through this switch to the trunk line 2|4 extending to the second selector switch 200; the line conductor CI 25 being connectedv by way of RAISI, RC|53, C|35, the wiper ||-1 and the contact |44 to the line conductor C2I5; the line conductor CI21 being connected by way of RA|58, RC|59, CI31, the wiper II9 and the contact |46 to the line conductor C2I1; andl the release conductor C|26 being connected by way of RCISB, RAI54, C|38, the wiper I|8 and the contact |45 to the release conductor C2I6.

Also, upon operating, the switching through relay R|50 interrupts, at RAI5I and RC|52 and at RAI58 and RCI59' the operating circuit of the line relay R|20, thereby interrupting the path for transmitting dial tone to the receiver of the telephone instrument at the substation A. At RAI51, the relay RI50 interrupts the operating circuit of the hold relay R|30, and, at RA|51. it interrupts a point in the circuit for energizing the release magnet MI86. When the line relay R|20 restores, it interrupts, at RA|2| and RCI22, a further point in the operatingcircuit of the hold relay RI30 and prepares, at RAI2I and RCI23, a point in the circuit for energizing the release magnet MI 86. Shortly thereafter, the hold relay RI30 restores and interrupts, at RAI 32 and RCI33,-a further point in the circuit for energizing in series the control relay R|40 and the vertical magnet MI84, and prepares, at RAI32 and RCI34, a further point in the circuit for energizing the release magnet MI86. Also, upon restoring, the hold relay R|30 removes ground potential at RA|3I from the release conductor C|26, rendering the release of the switch train including the operated lrst selector switch |00 under the control of the second selector switch 200, ground being retained upon the release conductor C2|6 in the second selector switch 200 at this time in a manner more fully described subsequently. The switching through relay RI50 in the first selector switch |00, the nder switch |82, and the relay R|10 in the line circuit are retained in operated positions at this time by virtue of completed circuits including the grounded release conductor C2|6.

In the event all of the trunk lines terminating at the selected level of bank contacts lead to busy second selector switches, the rotary magnet M| will cause the wipers of the mechanism |29 to be stepped to their respective eleventh offnormal positions, at which time the rotary 01T- normal springs |81 are moved to the off-normal position. When this occurs, the circuit for energizing the switching through relay RI50 in series with the control relay R|60 is interrupted at the contact springs |88 and |89 in order to prevent operation of RI50, and the wiper ||8 connected to the conductor C|36 disengages the contact in the tenth contact set having ground potential thereon, in order toxprevent reoperation of the control relay RI60 and the consequent rotation of the wipers into a further position by the rotary magnet MI85. Also, at the contact springs |9| and |92, dial tone is removed from the loop extending to the receiver cf the telephone instrument at the substation A, while, at the contact springs |90 and |92 of the rotary off-normal springs |81, busy tone current is returned to the receiver of the telephone instrument at the substation A. This circuit extends from the grounded winding of a busy tone current transformer, not shown, to the busy tone conductor C|39. and by way of the contact springs |90 and |92, the lower winding of R|20, RCI59', RAI58, C|21, the finder switch |82, and through the receiver of the telephone instrument included in the bridge across the calling line |0I, the finder switch |82, C|25, RAI5I, RCI52 and the upper winding of R|20 to grounded battery. The resulting busy tone indicates to the calling subscriber that the desired connection can not immediately be completed.

The calling subscriber. upon receiving busy tone, may initiate the release of the operated switch train by restoring his receiver to its hook to'open the loop circuit over which the line relay R|20 is held operated. This relay, upon restoring, interrupts, at RA|2| and RCI22, the operating circuit of the hold relay R|30 and prepares, at RA|2| and RC|23, a point in the circuit for energizing the release magnet MI86. Shortly thereafter, the hold relay R|30 restores and removes ground potential, at RAI 3| from the release conductor C|26, whereupon the operated finder switch |82 is restored to its idle position and the operated relay RI 10 in the line circuit |80 is restored to its idle position. Also, upon restoring, the hold relay RI 30 completes at RAISZ and RC|34 the circuit for energizing the release magnet M|86, this circuit extending from ground at RA|51 by way of RAIZI, RCl23, RAI32, RCI, the contact springs and |96 of the vertical off-normal springs |91 and the winding of MI06 to battery. In response to the energization of the release magnet MISS, the wipers of the mechanism |29 are restored to normal, at which time the rotary and vertical olf-normal springs |81;

and |91, respectively, are returned to normal. Thus, the partially completed connection is cleared out and the switches involved therein are released for further use.

Continuing now with the operation of the apparatus to establish a connection to the called line |09 and assuming that the second selector switch 200 is selected by the first selector switch |00 to forward the connection, immediately the loop circuit is extended to the second selector switch 200, the line relay R2|0 in this switch is' energized. This circuit extends from the grounded release conductor C|26 by way of RCI, RA|54, C|36, the wiper ||8, the contact |45, the release conductor C2|6, the contact spring 264 of the rotary off-normal springs 263, the contact springs 214 and 215 of the vertical off-normal springs 269, the contact springs 261 and 268 of the rotary off-normal springs 263, the lower winding of R2l0, RC249, RA248, the line con# ductor C2|1 of the trunk line 2|4, the operated iirst selector switch |00, the operated finder switch |82, the bridge at the substation A across the line I 0I, the operatedfinder switch |82, the operated rst selector switch |00, the line conductor C2|5 of the trunk line 2I4, RA24I, RC242 and the upper winding of R2| 0 to battery. Upon operating, the line relay R2I0 completes, at Also, each time the bridge across the line con-v RA2II and RC2I2, an obvious circuit for energizing the hold relay R220, this circuit extendlng from ground Vat RA24`I by way of RA2II, RC2I2 and the winding of R220 to battery. The last-mentioned relay in turn operates to complete, at RA22I, an obvious path for applying multiple ground potential to the release conductor C2I6 of the trunk line 2 I4, thereby to hold the rst selector switch |00 and the finder switch l02 operated and to retain ground potential upon the control conductor CID'I of the calling line IOI after the hold relay RI30 in the rst selector switch |00 restores to remove ground at RAI3I from the release conductor CI26 aspreviously explained. This ground potential applied to the release conductor C2I6 also retains the relay RI'I0 in the line circuit |80 in operated position. At RA222 and RC223, an impulsing circuit is prepared for energizing in series the control relay R230 and the vertical magnet M260 in accordance with the impulses comprising the second digit dialed at the calling substation A. Also, the hold relay R220 completes, at RA225, a path for short-circuiting the contact springs 210 and 215, thereby rendering the energization of the line relay R2 I0 under the control of the grounded release conductor C2I6 and independent of the vertical off-normal springs 260.

The impulsing circuit for the control relay R230 and the vertical operating magnet M260 is completed at' the beginning of the rst impulse of the second series of impulses dialed at the calling substation A and when the line relay R2I0 restores, this circuit extending from ground at R A20'I by way of RAZI I, RC2I3, RA222. RC223, the winding of R230 and the winding of M260 to battery. When energized over the above-traced circuit, the vertical magnet M260 lifts the wiper carriage structure of the mechanism 2I0 one step or to a position opposite the rst level of bank contacts. Upon operating, the control relay R230 prepares at RA23I and RA233 a circuit, traced hereinafter, for energizing the control relay R250 and interrupts at RA233 and RC234 a point in the circuit for energizing the rotary magnet -M26I. taneously 'with this operation, the vertical offnormal springs 269 are operated to the off-normal position, thereby completing at the contact springs 210 and 2'II the circuit for energizing the control relay R250, this circuit extending from ground at RA22I by way of RA233, RA23I, the contact springs 2l0 and 2'II and the winding of R250 to battery. At the contactsprings 212 and 273 of the vertical off-normal springs 269. a circuit, traced hereinafter for energizing the release magnet M262 is prepared, and the contact springs 270 and 275, short-circuited by the armature RA255 of the hold relay R220, are moved out of contact with each other. During each impulse of the second series of impulses, the vertical magnet M260Y is energized in the abovedescribed manner so that', at the conclusion of the second digit, the wiper carriage structure of the mechanism 2I8 stands in a position opposite the level of bank contacts terminating the group of trunks outgoing to the final selector or connector switches at which the called line |00 terminates. During the second series of impulses, each time the bridge across the line conductors CI02 and CI03 at the substation A is interrupted, the line relay R2I0 restores, causing the control relay R230 and the vertical magnet M200 to' be energized, as previously explained.

Simulductors CI02 and C503 at the substation A is completed, the line yrelay R2I0v reoperates and completes the energizing circuit of the hold relay R220. Thus, the hold relay R220 and the control relay R230 remain operated during the impulsing period, due to their slow-to-release characteristic. Upon operating, the control relay R250 prepares at RA25I a point in the circuit for energizing the rotary magnet M26I and completes at RA252 a locking circuit for itself, the latter circuit extending from ground at RANT by way of RA252, the contacts controlled by the armature 26| of the rotary magnet M26I, the contact springs 210 and 2'II and the winding of R250 to batt'ery.

With the wiper carriage structure of the switching mechanism 2I0 standing opposite the level of bank contacts terminating the group of trunk lines leading t'o the respective final selector or connector switches of the group including the iinal selector or connector switch 300, the second selector switch 200 is automatically conditioned to search for an idle one of these trunk lines. second series of impulses, the control relay R230 restores to interrupt. at RA23I and RA233. the original operating circuit of the control relay R250 and to complete, at RA233 and RC230, the

circuit for energizing the rotary magnet MZSI, I

the latter circuit extending from ground at RA22I by way of RA233, RC230, RA25I and the winding of M26I to battery. Upon being energized over the above-traced circuit, the rotary magnet M2M steps the wipers of the mechanism 2I8 into engagement with the first set of contacts in the selected level and simultaneously interrupts, at 20V, the locking circuit of the control relay R250, which latter relay restores and interrupts, at RA25I, the operating circuit of the rotary magnet M26I and interrupts, at RA252, a further point in its locking circuit. When the rotary magnet M26I restores, it prepares a point..

at 26I, in a circuit for testing the idle or busy condition of the trunk line terminating at the engaged first set of contacts in the selected level. If this rst trunk line is busy, ground potential appears upon the release conductor thereof. thereby causing the control relay R250 to be reoperated over a circuit extending from the grounded release conductor by way of the conductor C29I.RA230, RC205. 26|'. the contact springs 210 and 2II and the winding of R250 to battery. It is noted that the ground potential appearing upon the conductor 029i and extending by way of RAM@ and RC205 to the upper terminal of the switching-through relay R200 causes this relay to be short-circuited, as the lower terminal of the Winding thereof is connected by way of the contact springs 265 and y2550` of the rotary olf-normal springs 263 to the grounded release conductor CZIG, thereby preventing operation of this relay. Upon reoperating, the control relay R250 completes, at RA25I the circuit for energizing the rotary magnet MZSI and completes, at RA252, its locking crcuit. Upon operating, the rotary magnet M26I steps the wipers of the mechanism 2| 8 into engagement with the second set of contacts in the selected level and simultaneously interrupts at 26| the locking circuit of the control relay R250. whereupon R250 restores and interrupts, at RA25I, the operating circuit of the rotary magnet M26I and interrupts, at RA252, a further point in its locking circuit. The rotary magnet M26I then restores and completes, at 20|', the

Thus, shortly following the end of the circuit for testing the idle or busy condition of the trunk line terminating at the second set of contacts in the selected level. If the second trunk is busy, ground potential appears upon the release conductor thereof, causing the control relay R250 to be reoperated, as previously explained, in order to cause the rotary magnet M26| to be rcoperated, thereby stepping the wipers of the mechanism 2|8 into engagement with the third set of contacts in the selected level. Thus, the rotary magnet M26| operates to rotate the wipers of the mechanism 2|8 step by step over the contact sets in the selected level until' they are moved into engagement with a trunk line extending to an idle connector switch.

Assuming that the connector switch 300 is the iirst idle connector switch in the selected level, when the wipers of the mechanism 2|8 engage the set of contacts terminating the trunk line 295 extending to the connector switch 300, further operation of the rotary magnet M26| is interrupted, as no potential whatever appears upon the release conductor C291 of the trunk line 295. Thus, the switching through relay R240 is not short-circuited but is energized in series with the control relay R250 over a circuit extending from the grounded release conductor C2|6 by way of the contact springs 264 and 265, the winding of R240, 26|', the contact springs 210 and 21|, and the Winding of R250 to battery. When the switching through relay R240 is energized over the above-traced circuit, it is operated; while the control relay R250, which is of the marginal type, is not operated, due to the high series resistance of this circuit. Upon operating, the switching-through relay R240 connects the line conductors C2|5 and C2|1 of the trunk line 2M to the conductors C290 and C292, respectively, and connects the release conductor C2|6 of the trunk line 2|4 to the conductor C29|. At

this time the trunk line 2|4 extending to the' second selector switch 200 is connected through the selector switch 200 to the trunk line 295 extending to the connector switch 300; the line conductor C2|5 being connected by way of RA24I, RC243, C290, the wiper 216 and the contact 280 to the line conductor C296; the conductor C2|1 being connected by way of RA248, RC249, C292, the wiper 219 and the contact 282 to the'line conductor C298; and the release conductor C2|6 bein'g connected by way of RC246, RA244, C29I, the wiper 211 and the contact 28| to the release conductor C291.

Also, upon operating, the switching-through relay R240 interrupts, at RA24| and RC242 and at RA248 and RC249', the operating circuit of the line relay R2|0. At RA241, the relay R240 interrupts the operating circuit of the hold relay-R220 and, at RA241, it interrupts a point in the circuit for energizing the release magnet M262. When the line relay R2|0 restores, it interrupts, at RA2|| and RC2| 2, a further point in the operating circuit of the hold relay R220 and prepares, at RA2|| and RC2I3, a point in the circuit for energizing the release magnet M262. Shortly thereafter, the hold relay R220 restores and interrupts, at RA222 and RC223, a further point in the circuit for energizing in series the control relay R230 and the vertical magnet M260, and prepares, at RA222 and RC224, a further point in the circuit for energizing the release magnet M262. Also, the hold relay R220 removes ground potential at RA22| from the release conductor C2|6, rendering the release of the switch train including the operated second selector switch 200 under the control of the connector switch 300, ground being retained upon the release conductor C291 in the connector switch 300 at this time in a manner more fully described subsequently. The switching through relays RI50 and R240 in the first and second selector switches |00 and 200, respectively, the nder switch |82, and the relay RI10 in the line circuit |80 are retained in operated positions at this time by vrtue of completed circuits including the grounded release conductor C291.

In the event all of the trunks terminating at the selected level of bank contacts lead to busy connector switches, the rotary magnet M26| will cause the wipers of the mechanism 2|8 to be stepped to their respective eleventh off-normal positions, at which time the rotary `bili-normal springs 263 are moved to the off-normal position. When this occurs, the circuit for energizing the switching through relay R240 in series with the control relay R250 is interrupted at the contact springs 264 and 265, in order to prevent operation of R240, and the wiper 211 connected to the conductor C29| disengages the contact in the tenth contact set having ground potential thereon in order to prevent reoperation of the control relay R250 and the consequent rotation of the wipers into a further position by the rotary magnet M26I. At the contact springs 266 and 268 of the rotary off-normal springs 263, a path is completed for returning busy tone current to the receiver of the telephone instrument at the substation A. Thisl path extends from the ungrounded terminal of the grounded winding of a busy tone current transformer, not shown, to the busy tone conductor C218, and by way of the contact springs 266 and 268, the lower wind ing of R2|0, RC249, RA248 and C2I1 to the finder switch |82, and through the receiver included in the bridge across the calling line |0|, the finder switch |82, C2l5, RA24I, RC242 and the upper winding of R2|0 to grounded battery. The resulting busy tone indicates to the calling subscriber that the desired connection can not immediately be completed.

The calling subscriber, upon receiving busy tone, may initiate the release of the operated switch train by restoring his receiver to its hook to open the loop circuit over which the line relay R2|0 is held operated. This relay, upon restoring, interrupts, at RA2|| and RC2|2, the operating circuit of the hold relay R220 and prepares, at RA2|| and RC2| 3, a point in the circuit for energizing the release magnet M262. Shortly thereafter, the hold relay R220 restores and removes ground potential, at RA22|, from the release conductor C2|6, whereupon the iirst selector switch |00 and the operated finder switch |82 are restored to their idle positions and the operated relay RI10 in the line circuit |80 is restored to its idle position. Also, upon restoring, the hold relay R220 completes, at RA222 and RC224, the circuit for energizing the release magnet M262, this circuit extending from ground at RA241 by way of RA2||, RC2l3, RA222, RC224, the contact springs 212 and 213 of the vertical off-normal springs 269 and the winding of M262 to battery. In response to the energization of the release magnet M262, the wipers of the mechanism 2|8 are restored to normal, at which time the rotary and vertical off-normal springs 263 and 269, respectively, are returned to normal. Thus, the partially completed connec- 75 2,189,400, 'tion is cleared out. and the switches involved therein are released for further use.

Continuing now with the operation of the apparatus to establish a connection to the called line |09 and assuming that the connector switch 300 .is selected by the second selector switch 200 to forward the connection, immediately the loop circuit is extended to the connector switch 300, the line relay R320 in this switch is energized. This circuit extends from the grounded release conductor C291 by way offthe contact springs 388 and 381 of the vertical off-normal springs 385, ythe lower winding of R320, RC3I6, RA3I4, the line conductor C298 of the trunk line 295, the operated switch train including the second selector switch 200, the first selector switch |00, the finder switch |82 and the bridge across the calling line |0I, the operated switch train, the line conductor C296 of the trunk line 295, RA3I2, RC3|3 and the upper winding of R320 to battery. Upon operating, the line relay R320 completes, at RA32I and RC322, an obvious circuit for energizing the hold relay R330. The last-mentioned relay vin turn operates to complete,l at RA33I, an obvious path for applying multiple ground potential to the release conductor C291 of the trunk line 295. thereby to hold the switch train operated and to retain ground potential upon the control conductor CI01 of the calling line IOI after the hold relay R220 in the second selector switch 200 restores to remove ground at RA22I from the release conductor C2IG, as previously explained. At RA332, the hold relay R330 prepares a circuit, traced hereinafter, for energizing the switching through relay R380 and, at RA333 and RC334, an impulsing circuit is prepared for energizing in series the control elay R340 and the vertical magnet M390 in accordance with the impulses comprising the third digit dialed at the calling substation A.

The impulsing circuit for the control relay R340 and the Vertical magnet M390 is completed at the beginning of the rst impulse of the third series of impulses dialed at the calling substation A- and when the line relay R320 restores. this circuit extending from ground at RA32| by way of RC323, RA333, RC334, the contacty springs 398 and 395 of the vertical off-normal springs 393. the winding of R340, and the winding of M390 to battery. When energized over the above-traced circuit, the vertical magnet M390 lifts the wiper carriage structure of the mechafnism 399 one step or to a position opposite the first level of bank contacts. Upon operating, the control relay R340 preparesat RA34| and RC342 an alternative impulsing circuit, traced `hereinafter, for energizing in series the control relay R340 and the vertical operating magnet M390 and interrupts lat RA34I and RC343 a point in the circuit for energizing in parallel the control relay'R310 and the-rotary magnet M39I. Simultaneously with this operation, the vertical off-normal springs 385 are operated to `the offnormal pcsition, thereby interrupting at the contact springs 381 and 388 the original operating circuit of the line relay yR320 and completing at the contact springs 386 and 381 an alternative circuit for energizing the linerelay R320, the latter circuit extending from ground at the contact spring 385 by Way of the contact spring 381, the lower winding of R329, RCSIS', and RA3|4 to the line conductor C298 of the trunk line 295, through the operated switch train and the bridge, at the substation A, across the calling line lili to the line conductor C298 of the trunk line 295,

and from the line conductor C296 by way of RA3I2, RC3I3 and the upper winding of R320 to battery. Also, the verticaboff-normal springs 393 are moved to the off-normal position, thereby interrupting, at the contact springs 398 and 398, the original impulsing circuit for energizing in series the control relay R340 and the vertical magnet M390, preparing at the contact springs 391 and 398 a point in the alternative impulsing circuit for energizing in series the control relay R340 and the vertical magnet M390, and preparing at the contact springs 394 and 395 a point in a circuit, traced hereinafter, for energizing the release magnet M392. During each impulse of the third series of impulses, the vertical magnet M390 is energized in the above-described manner so that, at the conclusion of the third digit, the wiper carriage structure of the mechanism 399 stands in a position opposite the level of bank contacts at which are terminated the lines of the ten-line group including the called line |09. During the third series of impulses, each time the bridge across the line conductors CI02 and CI03 at the substation A is interrupted, the line relay R320 restores, causing the control relay R340 and the vertical operating magnet M390 to be energized as previously explained. After the first impulse the circuitfor energizing in series the control relay R348 and the vertical magnet M390 extends from ground at RA32I by way of RC323, RA333, RC334, the contact springs 398 and 391, RA34I, RC342, the Winding of R340 and the winding of M90 to battery. Also, each time the bridge across the line conductors CI02 and CI03 at the substation A is completed, the linerelay R320 reoperates and completes the,en\ergizing circuit of the hold relay R330. Thus, theshold relay R330 and the control relay R340 remain operated during the impulsing, due to their(y slow-to-release characteristics.

With the wiper carriage structure of the switch mechanism 399 standing opposite the level of bank contacts at which are terminated the lines of the ten-line group including the called line |09, the connector switch is conditioned to rotate the Wipers step by step in accordance with the impulses comprising the fourth and final digit dialed at the calling substation. Thus, shortly following the end of the third series of impulses, the control relay R340 restores to interrupt, at RAMI and RC342, the alternative impulsing circuit for energizing in series the control relay R349 and the vertical magnet M390 and prepares, at RA34| and RC343, a circuit, traced hereinafter, for energizing in parallel the control relay R310 and the rotary magnet M39l. At the beginning of the first impulse of the fourth series of impulses dialed at the calling substation A and when the line relay R320 restores, the impulsing circuit for energizing in parallel the control relay R310 and the rotary magnet M39! is completed, this circuit extending from ground at RA32I by wayv of RC323, RA333, RC334, the contact springs f tion of the called line |09, and completes, at RA315, a path in the circuit for energizing in parallel the control relay R310 and the rotary magnet M39| short-circuiting the contact controlled by the armature RA364 of the busy relay R360. When the rotary magnet M39| is energized over the above-traced circuit, the Wipers of the mechanism 399 are rotated into engagement with the contacts of the first contact set in the level of contact-sets at which the called line |09 terminates. The control relay R310 and the rotary magnet M39I are repeatedly energized over the above-traced circuit in accordance with the series of impulses comprising the fourthldigit dialed at the substation A, whereupon the wipers of the mechanism 399 engage the contact set in the previously selected level at which the called line |09 terminates. At this time the wipers 316, 311 and 318 of the mechanism 399 engage the contacts 3I1, 3|8 and 3I9, respectively, at which the conductors CIIO, CII5 and CII2, respectively, of the called line |09 terminate. The line comprising the conductors CII 0, CII5 and CI I2 also constitutes na connecting line, the line conductor CI I0 and CI I2 thereof being associated in the line circuit I8| with the line conductor CI I0 and CI II of the subscriber line |09 in a manner more fully explained below.

In the event the called line |09 is busy, ground potential appears upon the control conductor CI I5 thereof, whereupon the busy relay R360 is energized, this circuit extending from the grounded control conductor CII5 of the called line |09 by way of the contact 3I8, the wiper 311, the conductor C368, RA31I, RA312, and the winding ofR360 to battery. Whenv energized over the above-traced circuit, the busy relay R360 operates to interrupt, at"`RA36I and RC362, a point in the circuit for-'en`ergizing the switching-through relay R380 and prepares, at RA36I and RC363, a point in a circuit, traced hereinafter, for locking the busyrelay R360 in operated' position. Also, the busy relay R360 interrupts, at RA364, a point in the original impulse circuit for energizing in parallel the control relay R310 and the rotary magnet M39I and completes, at'RA365 and RC3G6, a circuit for returning busy tone current to the calling substation A. The latter circuitlextends from pulsating ground upon the busy conductor C366' by Way of RC366, RA365C348, the voice coupling condenser 346, to the line conductor C298 of the trunk line 295,` from the line conductor C298 through the operated switch train, the bridge including the receiver of the telephone instrument across to calling line IOI at the vcalling substation A and back through the operated switch train to the line conductor C296 of the trunk line 295, and from the line conductor C296 by way of the voice coupling condenser 345, C341 Vand the upper Winding of R3I0 to battery. Shortly following the termination of the fourth series of impulses, the control relay R310 restores, due to its slow-to-releas characteristic, and completes, at RA312 and C314, a locking circuit for the busy relay R360, this circuit extending from ground at RA332 by Way of RA36I, RC363, RC314, RA312 and the winding of R360 to battery. Upon restoring, the. con" trol relayR310 also completes, at RA31I and RC313, a point in the circuit, traced hereinafter, for energizing the switch-through relay R380, this circuit being interrupted, at RA36| and RC362, atY this time. The resulting busy tone indicates to the calling subscriber that circuit I8I individual to the called line |09, causing the busy relay to be short-circuited and causing the switching-through relay R380 to be energized, after the control relay R310 restores. The circuit for energizing R380 extends from ground at RA332 by way of RASBI, RC382, the upper winding of R380, RC313, RA31I', the conductor C368, the wiper 311, the contact 3| 8, the control conductor CI|5 of the called line l|09 and the intermediate winding of the combined line and cut-ofi relay RI65 in the line circuit IBI to battery. When energized over the abovetraced circuit, the switching-through relay R380 operates and prepares, at RA389 and RA384, a circuit for projecting ringing current'over the called line to the called substation B, thereby to operate the ringer, disposed thereat, completes, at RA382, a locking circuit for itself extending from the grounded release conductor C291 by Way of RA382 and the lower winding of R380 to battery, and interrupts, at RA383, aI

point in the impulsing circuit for energizing in parallel the control relay R310 and the rotary magnet M39I. Also, when the switching-through relay R380 operates, it completes, at RA38I, a circuit for fully energizing the combined line and two-step cut-off relay RI65 in the line circuit battery. When the relay R|65 is energized over i the last-mentioned circuit, it fully operates to interrupt, at RAIBS and RA I61, the connections between the upper and lower windings, respectively, thereof and the line conductors CI I0 and CHI of the called line |09. Also, the relay RIS5 completes, at RAISB, a point in the circuit for projecting ringing current over the called line |09 to the substation B. This circuit extends from pulsating ground on the ringing conductor C351 by way of RC351, RA355, the conductor C348', RA384, the conductor C369, the wiper 318, the contact 3|9, the line conductor CI I2, RAI68, the line conductor CI I I, the ringer bridged across the line conductors CI I I and CI I0 of the called line |09 at the called substation wiper 316, the conductor C361, RA389, the conductor C341', RA35I; RC352 and the upper winding 6i the ring cut-orf relay R350 to battery. When the ringer is energized over the abovetraced circuit, the called party is given an audible signal that the substation B is being called. A portion of this current is returned through the condenser 355 and the voice coupling condensers 345 and 346 over the loop extending to the calling substation A, in order to indicate to the calling party that the called substation B is being rung. The relay RI65 also completes, at RAI69, a point in a circuit for marking the called B, the line conductor CI|0, the contact 3| 1, the

tending to the calling substation A and the loopl line |09 as busy in the group of finder switches including `the finder switch |82 and interrupts,

at RA| 64, a'point in a circuit extending by way of the distributor start conductor C|11 for causing the distributor |83 to operate, thereby preventing an idle finder-selector link from being assigned to the called substation B when the substation B is called.

When the call is answered at the called substation B by removing the receiver of the telephone instrument thereat from its associated hook, a low resistance circuit is completed for energizing the ring cut-01T relay R350, this circuit being the same as the circuit for projecting ringing current to the substation B, except for the low resistance path bridged across the line conductors Cl I0 and CI Il of the called line |09 at the substation B in parallel with the ringer thereat. Shortly thereafter, the ring cutoi relay R350, being of the slow-to-operate type, is operated to interrupt, at RA355 and RC351, the circuit for projecting ringing current to the called substation B; to complete, at RA354, a locking circuit for itself extending from the grounded release. conductor C291 by way of RA354, and the lower winding of R350 to battery; to interrupt, at RA35| and RC352 and at RA355 and RC351, the original operatingcircuit of R350; and to complete, at RA35| and RC353 and at RA355 and RC356, a loop circuit extending to the called substation B. When the loop circuit is extended to the called substation B; the battery reversing relay R3|0 is energized, this circuit extending from ground at the upper terminal of the lower winding of R3|0 by way of the lower winding of R3|0 to the line conductor C348 and from the line conductor C348 by way of RC356, RA355, C348', RA384, C369, the wiper 378, and the contact 3|9 to the line conductor CH2 of the called' line |09, through the telephone instrument bridge-d across the line conductors of the line |09 at the substation B to the line conductor Cllllv of the called line- |09, and from the line conductor CI I0 by way of the contact 3|1, the wiper 316, C361, RA389, C341', RA35I, RC353, the line conductor C341 and the upper winding of R3|0 to battery. When energized over the abovetraced circuit, the battery reversing relay R3|0 operates to reverse the batte-ry applied to the line conductors C296 and C298 of the trunk line 295 extending by way of the operated switch train to the calling substation A, this circuit extending from ground at the contact spring 386 of the vertical oir-normal springs 385 by way of the contact spring 381, the lower winding of R320, RASII, and RA3|2 to the line conductor C296,vfromrthe line conductor C296 through the operated switch train and the telephone instrument at the calling substation A to the line conductorC298, and from the line conductor 'C298 by way of RA3I4, RA3I5 and the upper Winding of R320 to battery. This reversal of battery potential as applied to the)line conductors C296 and C298 of the trunk line 295 extending by way of the operated switch train to the calling substation A may beutilized to operate a register, coin collecting device, etc., associated with the calling substation A.

At this time, talking battery is supplied to the loop extending to the calling substation, A through the windings of the line. relay R320 in the connector switch 300 and talking battery is supplied to the loop extending to the called substation B through the windings of the battery reversing relay R3|0. Moreover, the loop exstore. K relay R240 completes, at RA241, the circuit for extending to the called substation B are con-l nected for communication therebetween by the voice coupling condensers 345 and 346 interposed,

respectively, between the line conductors C296 and C341 and the line conductors C298 and C348.

The release of the established connection in-y cluding the operated switch train is under the control of the calling party. established connection, the calling party replaces the receiver of the telephone instrument at the substation A upon its associated hook,

thereby interrupting the loop extending from the substation A and including the line relay R320. When this loop is interrupted, the line relay R320 restores to interrupt, at RA32| and'RC322, the operating circuit of the hold relay R330, and to prepare, at RA32| and RC323, a point in a circuit, traced hereinafter, for energizing the release magnet M392. Shortly thereafter, the hold relay R330 restores and interrupts, at RA33| the path for applying ground potential to the In releasing thel release conductor C291 of the trunk line 295, and

completes, at RA333 and RC335, the circuit for energizing the release magnet M392, the latter.

circuit extending from ground at RA32| by way of RC323, RA333, RC335, the contact springs 395 and 394 of the vertical off-normal springs 393 and the winding of M392 to battery. When the release magnet M392 is energized over the abovetraced circuit, the Wiper carriage structure of -the mechanism 399 is returned to normal, thereby moving the vertical oil-normal springs 393 into normal position. When the vertical off-normal springs 393 are moved into normal position, the circuit for energizing the release magnet M392 is interrupted at the contact springs 394 and 395. When the wiper carriage structure mechanism 399 is returned to normal position, the wipers 316, 311 and 318 disengage the contacts 3|1, 3| 8 and 3|9, respectively, thereby interrupting the loop extending to the called substation B. When the loop extending to the called substation B is interrupted, the operating circuit of the battery reversing relay R3|0 is interrupted, causing R3|0 to restore. When ground potential is removed from the release conductor C291, the locking circuit for the switching through relay R380 and the locking circuit for the ring cut-off relay R350 are. interrupted, causing these relays to restore. At this time, the connector switch 300 is completely released.

Also, when the loop extending to the called sub-station B is interrupted, the circuit for energizing the relay R|65 is interrupted, causing this relay to restore. The removal of ground potential from the release conductor C291 simultaneously removes ground from the release con ductor C2|6 and C|26 connected thereto. When ground potential is removed from the release conductor C2|6, the circuit 'for energizing in series, the control relay R250. and the switching through relay R240 in the second selector-switch 209 is interrupted, causing the relay R240 to re- Upon restoring, the switching through energizing the release magnet M262, this circuit.

tion, causing the vertical off-normal springs 269 to be returned to normal position in order to interrupt, at the contact springs 212 and 213, the circuit for energizing the release magnet M262. At this time, the second selector switch 200 is completely released.

When ground is removed from the release conductor C|26, the circuit for energizing in series the control relay RI60 and the switching through relay R|50 in the rst selector switch |00 is interrupted, causing the relay R|50 to restore. Upon restoring, the switching through relay R|50 completes, at RAIST, a circuit for energizing the release magnet MllBG, this circuit extending from ground at RAI 51 by way of RAI2I, RCI23, RAI32, RCl34, the contact springs |95 and |96 of the vertical off-normal springs |91' and the winding of M|86 to battery. When the release magnet MISE is energized over the above-traced circuit, the wiper carriage structure of the mechanism |29 is returned to normal position, causing the vertical oir-normal springs |91 to be returned to normal position to interrupt, at the contact springs |95 and |96, the circuit for energizing the release magnet M|86. At this time the flrst selector switch is completely released. Also, when ground potential is removed from the release conductor C|26, the iinder switch |82 is released and the circuit for energizing the relay RI'IU in the line circuit |80 is interrupted, causing the relay R| to restore, in order to release the line circuit |80. At this time the established connection between the calling substation A and the called substation B is completely cleared out and all of the equipment included therein is in readiness for further use.

In accordance with one feature of the invention and as pointed out above, each of the line circuits is connected and arranged to, prevent the presence of a potential diierence between the contacts in the contact sets in the contact banks of the Strowger nder and connector switches terminating the subscriber line individual to the line circuit when the Aassociated substation is idle. Specically, the line circuit |8| is connected and arranged to prevent the presence of potential diierence between the contacts in the contact sets in the contact banks of the nder and connector switches |82 and 300, respectively, terminating the subscriber line |09 individual to the line circuit |8|' when the associated substation B is idle. The connection and arrangement of the line circuit |8| will best be understood from the description of the operation thereof appearing hereinafter, while an understanding of the operation of the improved line circuit |8| will be facilitated by considering the arrangement and operation of a conventional line circuit of this type. In a telephone system of the form illustrated in Figs. 1, 2 and 3, utilizing a conventional line circuit of this type, the line conductorCI of the subscriber line |09 would be connected rigidly to the line conductor CI I2 thereof. Thus, when the substation B is idle, ground potential wouldv be present upon the line conductor C| |2, this path extending from ground at the upper terminal of the lower winding of the relay RISE by way of RAl'l to the line conductor CH2; and the negative potential of grounded battery would be present upon the line conductor Cl I0 of the subscriber line |09,.this path extending from the negative terminal of grounded battery at the lower terminal of the intermediate winding of the relay RI65 by way of the winding of R|65, the control conductor C||5, the upper winding of R|65 and RA|66 to the line conductor CI I0. Hence, a potential difference would be present between the contacts to which the line conductors Cl l0 and CI |2 are respectively connected in the contact sets in the contact banks in the finder and connector Switches |82 and 300, respectively, terminating the subscriber line |09 individual to the line circuit |8I when the associated substation B is idle. In a similar manner, a potential diierence would be present between the contacts in the contact sets n the multipled contact banks of the other nder and connector switches in the groups including the nder and connector switches |82 and 300, respectively. Likewise, a potential difference would be present between the contacts in the contact sets in the contact banks in all of the nder and connector switches terminating idle subscriber lines.

On the other hand, in a telephone system of the form illustrated in Figs. 1, 2 and 3, utilizing the improved line circuit |8|, the line conductor CI of the subscriber line |09 is normally disconnected from the line conductor CH2 thereof and is connected thereto by the contacts associated with the armature RA|68 of the relay R|65 included in the line circuit |8| only when the substation B is busy. Thus, when the substation B is idle, no potential whatever is present upon the line conductor CH2 as the armature RA|68 of the relay RI 65 occupies a restored position; and only negative potential of grounded battery appears upon the line conductor CIIU of the subscriber line |09, this path extending from the negative terminal of grounded battery at the lower terminal of the intermediate Winding of R|65 by way ofthe intermediate winding of RI'65, the ,control conductor C||5, the upper winding of RI fand RA|66 to the line conductor CIID. Hence, no ipotential difference whatever is present between`tl contacts to which the lineconductors C||0 and CH2 are respectively connected in the contact sets in the contact banks in the finder and connector 'switches |82 and 300, respectively, terminating the subscriber line |09 individual to the line circuit |8|, when the associated substation B is idle. Accordingly, when improved line circuits of the form of that indicated at |8| are utilized in a telephone system of this character, a potential difference appears between the contacts in the contact sets in the Contact banks in the nder and connector switches only when a substation is busy, at which time the potential difference between the contacts mentioned is desirable in order toaccomplish the functions of the apparatus included in the teleph'one system in the manner described above.

In accordance with another feature of the invention, each of the second selector switches is connected and arranged to prevent the presence of a potential difference between the contacts in the contact sets in the contact banks of the preceding Strowger switches to which the switchV is connected, when the switch occupies an idle position. Specically, the second selector switch 200 is connected and arranged to prevent the presence of a potential difference between the contacts in the contact set in the contact bank of the iirst selected switch H3G, when the second 70 selector switch 200 occupies an idle position. The connection and arrangement `of the second selector switch 20|) will best "ne understoodfrom the description of the operation thereof, appearing hereinafter, while an understanding of the vnector switch 300 will be facilitated by consideroperation of the improved second selector switch 200 will be facilitated by considering the arrangement and operation of a conventional second selector switch of this type. In a telephone system of the form illustrated in Figs. 1, 2 and 3, utilizing a conventional second' selector switch of this type, the lower terminal of the lower winding of the line relay R210 would be connected directly to ground, this path extending from ground at the lower terminal of the lower winding of AR210 by way of the lower winding of R210, RC249 and RA248 to the line conductor C211 of the trunk line 214; and the lower terminal of the upper winding of the line relay R210 would be connected to the line conductor C2 I 5 of the trunk line 214, this path extending from the negative terminal of grounded battery at the upper terminal of the upper winding of R210 by way vof the upper winding of R210, RC242 and RA241 to the line conductor C215. Hence, a potentialA difference would be present between the contacts in the contact set in the contact bank of the first selector switch 100 to which the trunk line 214 is connected when the conventional second selector switch occupies an idle position.

On the other hand, in a telephone system of the form illustrated in Figs. 1, 2 and 3, utilizing the improved second selector switch 200, the lower terminal of the lower winding of the line relay R210 is connected to the release conductor C216 ofthe trunk line 214, instead of being connected directly to ground, this path extending from the lower terminal of the lower winding of the line relay R210 by way of the Contact springs 268 and `261 of the rotary off-normal springs263 and the contact springs 215 and 214 of the vertical oinormal springs 269 to the release conductor C216 of the trunk line 214. Thus, when the improved second selector switch 200 occupies an idle position, no potential whatever is present upon the line conductor C211 and the release conductor C216, and the negative potential of grounded battery is present upon the line conductor C215. Hence, no potential difference is present between the contacts in the contact set in the contact bank in the rst selector switch 100 terminating the trunk line 214 when the second selector switch 200 occupies an idle position. Accordingly, when improved second selector switches of the form of that indicated at 200 are utilized in a telephone system of this character, la potential difference is present between the contacts in the contact sets in the bank contacts of the rst selector switches only when the second selector switches occupy operated'positions, at which time the potential is desirable in order to accomplish the functions of the second selector switches in the manner described above.

In accordance with another feature of the invention each of the connector switches is connected and arranged to prevent the presence of a potential dilT-erence between the contacts in the contact sets of the contact banks in the preceding Strowger switches to which the switch is connected, when the switch occupies an idle position. Specifically, the connector switch 300 is connected and arranged to prevent the presence of a potential difference between the contacts in the contact set in the contact bank of the second selector switch 200, when the connector switch 300 occupiesman idle position. Ihe connection and arrangement of the connector switch 300 will best be understood from the description of the operation thereof, appearingdaereinafter, while an understanding of the operation of the improved coning the arrangement and operation of a conventional connector switch of this type. In a telephone system of the form illustrated in Figs. 1, 2 and 3 utilizing a conventional connector switch of this type, the upper terminal of the lower winding of the line relay R320 would be connected directly to ground, thereby connecting groundl potential to the line conductor C298 of the trunk line 295, this path extending from ground at the upper terminal of the lower winding of R320 by way of the lower winding of R320', RC316 and RA314 to the line conductor C298; and battery at the lower terminal of the upper winding of R320 would be connected by way of the upper winding of R320, RC313 'and 12.11312 to the line conductor C296 of the trunk-line 295. Hence a potential diierence would be present etween the contacts in the contact set in the co/ tact bank of the second selector switch 200 to which the trunk line 295 is connected, when the conventional connector switch occupies an idle position. On the other hand, in a telephone system of the form illustrated in Figs. l, 2 and -3 utilizing the improved connector switch 300, the upper terminal of the lower winding of the line relay R320 is connected to the release conductor C291 by way of the contact springs 388 and 381 of the vertical off-normal springs 385, instead of being connected directly to ground, when the connector switch 300 occupies an idle position. Also the vertical off-normal springs 385 include a contact spring 380 which cooperates with the contact spring 381 to apply ground potential at the contact spring 386 to the upper terminal of the lower winding of the line relay R320, when the `con'y nector switch 300 occupies-an operated position., Thus, when the improved connector switch 300 occupies an idle position no potential Whatever is present upon the line conductor C298 and the release conductor C291, andthe negative potential of grounded battery is present upon the line conductor C296. Hence, no potential differencevis present between the contacts in the contact set in the contact bank of the second selector switch 200 terminating the trunk line 295, when the connector switch 300 occupies an idle position. Ac-

cordingly, when improved connector switches of the form of that indicated at 300 are utilized in a telephone system of this character, a potential diierence is present between the contacts in the contact sets in the contact banks of the second selector switches only when the connector switches occupy operated positions, at which time the potential is desirable in order to accomplish the functions of the connector switches in the e manner described above.

In accordance with further features of the invention each of the line circuits and each of the vsecond selector switches is connected and arranged to prevent hunting of the cut-01T relay in the line circuit and certain of the relays embodied in the second selector switch of an operated switch train during the release of the switch train. It will be understood that the connection and arrangement of the improved line circuits and the improved second selector switches constitute independent. means for preventing the hunting oi the relays noted, and only one of these means is required in order to eliminate the hunting of the relays. Specifically, the improved line circuit is connected and arranged to prevent the hunting of the cut-oir relay R110 therein and certainof the relays in the second selector switch 200, and the improved second selector switch 200 is constructed and arranged to prevent the hunting of the cut-oi relay R|10 in the line circuit |80 and certain of the relays in the second selector switch 2001 The connection and arrangement of the line circuit and the second selector switch 200 will best be'understood fromy a consideration of the operation thereof during the release of the switch train, appearing hereinafter, while an understanding of the operation of the improved line circuit. |80 and the improved second selector switch 200 will be facilitated by considering the arrangement and operation of a telephone system of this type including a reconstructed line circuit and a reconstructed second selector switch of the general types of the line circuit |80 andthe second selector switch 200, which reconstructed line circuit and second selector switch are not provided with the previously mentioned means for preventing hunting of the relays noted. In a telephone system of the form illustrated in Figs. 1, 2 and 3 utilizing a reconstructed line circuit similar to the line circuit |80, wherein the line conductor C|03 of the subscriber line |0| is directly connected to the line conductor C|04; and a reconstructed second selector switch similar to the second selector switch 200 wherein the lower terminal of the lower winding of the line relay R2|0 is connected to the release conductor C2|6 of the trunk line 2|4 only by way of thel contact springs 268, 261 and 264 of the rotary off-normal springs 263 and not including the contact springs 214 and 215 in the vertical off-normal springs 269 and the contacts controlled by the armature RA225 of the hold relay R220, hunting of the relays noted frequently appears, when the switch train is released.

Assuming that in this telephone system of the general form illustrated in Figs. l, 2 and 3 and embodying apparatus reconstructed in the manner set forth above, a call has been initiated at the substation A and the first digit has been dialed, causing the nder switch |82, the rst selector switch |00 and the second selector switch 200 to be seized and that the switch train is then released by replacing the receiver of the telephone instrument upon its associated hook at the substation A, the loop extending from the substation A and including the line relay R2| 0 in the second selector switch 200 is first interrupted, causing R2|0 to restore. Upon restoring the line relay R2|0 interrupts at RA2|| and RC2|2 the circuit for energizing the hold relay R220 and completes at RA2| and RC2|3 the circuit for energizing in series the control relay R230 and the vertical magnet M260. The vertical magnet M260 moves the wiper carriage struture of the mechanism 2|8 in the second selector switch 200 one step in the vertical direction, and the control relay R230 operates to complete at RA23| and RA233 the circuit for energizing the control relay R250, this latter circuit extending from ground at RA22| by way of RA233, RA23| the contact springs 210 and 21| of the vertical oir-normal springs 269 and the winding of R250 to battery. Also the control relay R230 completes at RA23| Vand RA233 a path for short-circuiting the switching through relay R240, this path extending from ground at RA22| by way of RA233, RA23I, 26|', the winding of R240, the contact springs 265 and 264 of the rotary off-normal springs 263 to the grounded release conductor C2|6 thereby causing the switching through relay R240 to remain restored. The hold relay R220, being of the slow-to-release type, restores shortly after the line relay R2 0 and removes ground potential at RA22| from the release conductor C2|6, causing the control relay R250 to restore. Upon restoring the hold relay R220 also interrupts at RA222 and RC223 the circuit for energizing in series the control relay R230 and the vertical magnet M260; the control relay R230 being of the slow-to-release type does not restore at this time. Simultaneously, ground potential is removed from the release conductor C|26 connected by way of RC|56, RA|54, Cl36, the wiper H8 and the contact |45 to the release conductor C2 6, from the control conductor C|05 connected by way of the nder switch |82 to the release conductor C| 26, and from the control conductor C|01 of the subscriber line |0| connected by way of RA|14 to the control conductor C|05.

When ground potential is removed from the release conductors C2|6 and C|26 and from the control conductors C|05 and C|01, the switching through relay R|50 in the first selector switch |00, and the cut-01T relay R|10 in the line circuit |80 tend to restore, and the nder switch |82 tends to release. Now suppose that the cut-off relay R|10 is fast to restore, the switching through relay R|50 is relatively slow to restore, and the finder switch |82 is relatively slow to release. rIhus, the cut-off relay R|10 is first to restore and upon restoring connects ground potential at RA|12 to the line conductor C|04 of the subscriber line |0|, this path extending from ground at the upper terminal of the lower winding of R|10 by way of the winding of R|10 and RA|12 to the line conductor C|04. This ground potential is fed forward through the finder switch |82 to the line conductor C|21 of the trunk line |24 and therefrom by way of RA|58, RCl59, C|31, the wiper 9, the contact |46, the line conductor C2|1 of the trunk line 2|4, RA248, RC249', the lower winding of R2I0, the contact springs 268, 261 and 264 of the rotary oli-normal springs 263 to the Yrelease conductor C2|5. This ground potential on the release conductor C2 I6 feeds forward by way of RA233, RA23 the contact springs 210 and 21| of the vertical off-normal springs 269 and the winding of R250 to battery. Hence, the lower winding of R|10, the lower winding of R2 |0 and the winding of R250 are energized in series relation. Due to the relatively high resistance of this series circuit the control relay R250 is not operated and the cut-0E relay R|10 is only partially operated, but the line relay R2|0 is fully operated.

Also, this .ground potential on the release conductor C2i 6 feeds back by Way of the contact |45, the wiper ||8, Cl36, RA|54 and RC|56 to the release conductor C|26, and therefrom by. way of the contact springs |88 and |89 of the rotary offnormal springs |81, the winding of R|50, the contact springs |93 and |94 of the vertical oilnormal springs |91 and the winding of RISO to battery. At this time the switching-through relay R|50 in the rst selector switch |00 is inits operated position due to the fact that this relay -is relatively slow to restore, as previously assumed. The switching-through relay R|50 and the control relay RI 60 receive sufficient energize.- tion over the above traced circuit to retain the relay R|50 in its operated position.

When the line relay R2|0 is operated over the series circuit previously traced, it completes at RA2| and RC2|2 the circuit for energizing the hold relay R220, which latter relay operates to apply ground potential at RA22| to the release conductor C2|6 to completely energize the relays R|50 and R|60 in series thereby to prevent the release of the rst selector switch |00. The application of ground potential to the release conductor C2 I 6 also prevents the release of the nder switch |82 and completely energizes the cut-oi relay R|10, causing the latter relay to operate. Upon operating the cut-off relay R|10 interrupts at RA|12 the path for applying ground potential to the line conductor CI04 of the subscriber line thereby interrupting the'above traced series circuit 'for energizing the lower winding of RI10, the lower winding of R2|0 and the winding of R250, causing R2|0 to restore. Shortly thereafter R220 restores and removes at RA22 ground potential from the release conductor C2|6.

Now, if the cut-off relay R| is fast to restore,

the switching through relay R|50 is relatively slow to restore, and the nder switch |82 is relatively slow to release, as previously assumed, the cycle above described will be repeated.' Thus the cut-oil relay R|10 in the line circuit |88 and the relays noted in the second selector switch 200 continue to hunt, causing the switch train to fail to release for further use, in spite of the fact that the substation A is idle.

In the telephone system illustrated in Figs. 1, 2 and 3 embodying the improved line circuit |00, one means for preventing the hunting of the cutof relay R| 10 in the line circuit |80 and the relays noted in the second selector switch 200 comprises an arrangement including an armaturelRA|13 provided on the cut-off relay R|10 and adapted to control contacts for connecting together the line conductors C|03 and C| 04 of the subscriber line i0 It is noted that the line conductor C|03 is normally disconnected from the line conductor C|04 and is connected thereto by the contacts controlled by the armature RA|13 only when the cut-off relay R|10 is fully operated. Thus in the improved line circuit |82 the hunting of the cut-off relay R|10 and the relays noted in the second selector switch 200 is positively prevented as ground potential is'not present on the line conductor C|04 after the cut-oir relay R|10 restores. Accordingly the previously traced series circuit extending from ground at the upper terminal of the lower winding of R|10 to battery at the upper terminal of the winding of R250 and including the lower winding of R|10, the lower Winding of R2|0 and the winding of R250 is not completed when the cut-01T relay R110 restores, regardless of the sequence of restoration of the cut-01T relay R|10 'in the line circuit |80 and the above enumerated relays in the second selector switch 200.

In the telephone system illustrated in Figs. 1, 2 and 3 embodying the improved second selector switch 200 another means for preventing the hunting of the cut-off relay R110 in the line circuit and the relays noted in the second selector switch 200 comprises the contact springs 214 and 215 of the vertical oli-normalv contact springs 269. It is noted that the lower winding of the line relay R2|0 is connected across the line conductor C2 l 'l and the release conductor C2 I8 of the trunk line 2|4 by way of RA248, RC249, the lower winding of R210, the contact springs 268 and 251 of the rotary oir-normal springs 263 and the contact springs 215 and 214 of the vertical oiT-normal springs 269 to the release conductor C2| 6, the contact springs 214 and 215 of the vertical off-normal springs 269 constituting a branch path in this connection. Also an armature RA225 is provided on the hold relay R220 which is adapted to control contact springs for short-circuiting the contact springs 214 and 215 of the vertical oft-normal springs 269, the contact springs controlled by the armature RA225 constituting a branch path in the previously mentioned connection. Thus, in the improved second selector switch 200, when the line relay R2| 0 restores and completes at RA2|| and RCZI3, the circuit for energizing in series the control relay R230 and the vertical operating magnet M260, the wiper carriage structure of the mechanism 2|8 is moved one step in the vertical direction, causing the vertical off-normal springs 269 to be moved into off-normal position, thereby interrupting, at the contact springs 214 and 215, one branch path in the connection between the line conductor C2|1 and the release conductor C2|6 including the lower winding of R2|0. The other branch path in this connection is interrupted when the hold relay R220 restores, thereby completely interrupting the previously traced series circuit extending from ground at the upper terminal of the lower winding of R| 10 to battery at the upper terminal of the winding of R250 and including the lower winding of RI10, the lower winding of R2 I0 and the winding of R250. The interruption of this series circuit positively prevents the hunting of the cut-01T relay R|10 in the line circuit |80 and the relays noted in the second selector switch 200, regardless of the sequence of restoration of the cut-off relay R in the line circuit |80 and the relays noted in the second selector switch 200.

Also it will be apparent that in the telephone system of the general form illustrated in Figs. 1, 2 and 3 and embodying apparatus reconstructed in the manner set forth above, the hunting of the relays noted in the 'manner described may be incident to the release of an established connection between two subscriber lines. For example, assuming that a release operation is performed at the substation A in order to release an established connection including the switches |02, |00, 200 and 300 between the substations A and B and that the switching through relay R240 in the second selector switch 200 restores together with the line .relay R|10 in the line circuit |80, upon the release of the connector switch 300 and prior to the'restoration of the switching through relay R|50 in the first selector switch |00 and the release of the finder switch |82 the above traced series operating circuit is completed at RAl12,4 at RAMI, RC242, and at RAZ48 and RC249. The relays noted hunt in the manner described above to prevent the release of the line circuit |80 and the switches |82, |00 and 200.

In the telephone system in Figs. 1, 2 and 3 embodying either the improved line circuit |80 or the improved second selector switch 200 this hunting of the relays noted incident to the release of an established connection between the substations A and B is positively prevented. I'he means comprising the contacts controlled by the armature RA|13 of the relay R|10 in the line circuit |80 prevents the previouslynoted series 

